Human tumor clonogenic assays (HTCA's) are used to test tumor specimens in vitro against chemotherapeutic drugs. Currently, use of HTCA's is limited to tumor specimens obtained at diagnosis or at overt relapse. An ideal situation would be totest metastatic cells prior to relapse before drug resistance is a factor. This situation is feasible by obtaining small numbers of tumor cells from the bone marrow of patients with a variety of solid tumors. The applicants have developed an HTCA that permits the clonogenic growth of tumor cellsfrom the bone marrow of breast cancer patients. The advantages of this HTCA method are: 1) testing can be performed at all stages of disease when metastatic cells are present in marrow; 2) sensitivity and specificity may be improvedbecause metastases in marrow may represent an aggressive clone of cellsthat are reflective of other systemic metastases; and 3) serial testing can be performed to identify emerging drug resistance as treatment proceeds. This marrow HTCA is also felt to be ideal for in vitro pharmacologic testing of new compounds against human metastatic tumor cells in their natural marrow microenvironment and for verification of tumor cell removal by various marrow-purging techniques.